The present invention relates to an automatic gain control apparatus which can prevent a peak part in white level of an image from being clipped when the image is transferred into an electric image signal, and any loss of information contained in the white level portion is thereby eliminated.
Heretofore, an image transfer curve having a knee slope fixed at a constant value of .gamma.=0.45 has been ordinarily used for a television broadcasting camera. However, when a camera of such a characteristic is used for obtaining picture of an object having a bright portion having a brightness of two to three times a 100% white set as a reference signal, the bright portion tends to reach a white clip level, thus resulting so-called "collapsed image". In other words, the contrast range of such a camera is narrower than the contrast range of an ordinary photographic film, and when a picture with a high contrast ratio, such as a person staying indoors against a background of a bright sky or window, is desired to be obtained, an adjustment of a camera to a value adapted for the contrast of the person tends to result an over exposure for the background, thus producing a picture having a background utterly white.
In a case where a photoconductive type image tube is used, the beam current of the same tube is ordinarily selected at two to three times of a maximum rated input level of the image tube in consideration of the resolution and the operational life of the same tube. Such a condition, however, is liable to cause disadvantageous effects termed "comet tail" and "blooming phenomenon".
To prevent such a disadvantage, an automatic beam optimizer (as disclosed in an article "Automatic beam current optimizer IC for improvement of latitude in hand-held TV camera" by Kotaro Wakui et al, NHK Laboratories Note Ser. No. 227, September 1978) has been used for automatically controlling the beam current in the image tube in accordance with an incident light quantity. By this optimizer, the light image can be transferred to an electric signal in a wide dynamic range without causing insufficient beam current even in a case where the incident light has an intensity as high as 16 times a reference incident light quantity.
However, in a present state of technique, an output electric signal obtained with a wide dynamic range for a high contrast object is processed in a circuit having a fixed knee characteristics as described above. For this reason, regardless of an input signal having a high white portion realized by the automatic beam optimizer, an output image signal having a white portion clipped and therefore collapsed has been obtained from the processing circuit. That is, the advantageous feature of the automatic beam optimizer has not been fully utilized.